Magnetic resonance imaging (MRI) is a medical imaging modality that can create pictures of the inside of a human body without using x-rays or other ionizing radiation. MRI uses a powerful magnet to create a strong, uniform, static magnetic field (i.e., the “main magnetic field”). When a human body, or part of a human body, is placed in the main magnetic field, the nuclear spins that are associated with the hydrogen nuclei in tissue water become polarized. This means that the magnetic moments that are associated with these spins become preferentially aligned along the direction of the main magnetic field, resulting in a small net tissue magnetization along that axis (the “z axis,” by convention). An MRI system also comprises components called gradient coils that produce smaller amplitude, spatially varying magnetic fields when a current is applied to them. An MRI system may use a shielded gradient coil that consists of inner and outer gradient coil assemblies potted together with a material such as epoxy resin. Typically, gradient coils are designed to produce a magnetic field component that is aligned along the z axis, and that varies linearly in amplitude with position along one of the x, y or z axes. The effect of a gradient coil is to create a small ramp on the magnetic field strength, and concomitantly on the resonant frequency of the nuclear spins, along a single axis. Three gradient coils with orthogonal axes are used to “spatially encode” the MR signal by creating a signature resonance frequency at each location in the body. Radio frequency (RF) coils are used to create pulses of RF energy at or near the resonance frequency of the hydrogen nuclei. The RF coils are used to add energy to the nuclear spin system in a controlled fashion. As the nuclear spins then relax back to their rest energy state, they give up energy in the form of an RF signal. This signal is detected by the MRI system and is transformed into an image using a computer and known reconstruction algorithms.
During a patient scan, the gradient coil(s) of the gradient coil assembly that produce the magnetic field gradients dissipate large amounts of heat. The heat produced by the gradient coils can cause an increase in the temperature of the MRI system's patient bore and the magnet warm bore. Heating of the patient bore may reduce the amount of RF power that can be transmitted during imaging which in turn can affect the efficiency of the MRI system. In addition, an increase in temperature of the patient bore can be uncomfortable for a patient and may even become dangerous unless safety interlocks are designed to prevent overheating. Minimizing any increase in temperature of the patient bore is important to MRI scanner efficiency and safety.
The heat produced by the gradient coils may be removed from the gradient coil assembly by a cooling assembly that may include, for example, a remote heat exchanger/chiller and cooling tube(s) positioned at a given distance from the heat conductors. For a shielded gradient coil, a cooling tube may be provided for the inner gradient coil assembly and a cooling tube may be provided for the outer gradient coil assembly. A liquid coolant, such as water, ethylene or a propylene glycol mixture, absorbs heat from the gradient coils as it is pumped through the cooling tubes and transports the heat to the remote heat exchanger/chiller. Heat may then be ejected to the atmosphere by way of the heat exchanger/chiller. The cooling tubes for the inner gradient coil assembly and the outer gradient coil assembly of a shielded gradient coil typically have a common cooling circuit, i.e., a common supply of chilled coolant. Generally, the chilled coolant is supplied at a temperature that removes the maximum amount of heat possible without causing condensation and also attempts to keep the patient bore temperature as close to room temperature as possible.
An increase in temperature of the magnet warm bore can also affect performance of the MRI system. A magnet warm bore surface is typically made of low magnetic permeability stainless steel, however, the stainless steel may have residual permeability, also known as ferromagnetism. According to Curie's law, the permeability of a ferromagnetic material changes as the temperature of the ferromagnetic material changes. A change in permeability of the magnet warm bore may result in a change in the intensity of the magnetic field, B0, generated by the magnet which can have a negative impact on image quality. As mentioned above, however, the cooling assemblies for a gradient coil are typically configured to remove as much heat as possible and minimize the patient bore temperature. Such cooling assemblies do not specifically address the problems caused by temperature changes in the magnet warm bore.
It would be desirable to provide a system for controlling the temperature of the magnet warm bore, in particular, for maintaining a constant warm bore temperature. It would also be advantageous to provide a cooling assembly that can both maintain a constant warm bore temperature and minimize the patient bore temperature.